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Friday, February 14, 2014

Warning: Reader Discretion Advised. This post contains discussion, photos and video of the naked interior of a Lego model as filmed by my wife. (People who searched for some of those keywords may be disappointed by this post.)
This part has been a bit tricky, so it's been a while since I posted. But it's starting to get there.

For Brickcon we pretty much had L3-G0's entire body built and ready, head moving, computers making noise, etc. All that is pretty cool, but he was just sitting on a table, and we want him to move, run around the room, scare the cats, and all that.

To do that, we need some sort of "frame" to hold all the Lego together. Something that won't shake apart on a rough sidewalk. We investigated using Lego, like Technic bricks, but L3-G0 is pretty big and heavy. The original model had a single Technic pin connecting the ankles, which clearly wouldn't work. So we settled on wood and aluminum.

This is from a test run of the frame. He's jury-rigged to move so that I can test how sound the design of the frame is. Hopefully I'll shake out any structural issues before he gets his skin on.

He's RC controlled, like a model airplane, with 2 channels being mixed using the radio's v-tail mode to control the left & right motors. So he ends up driving like a tank, though the control is with a single stick.

The motors are like what you'd use in a kid's electric scooter, and he's 8 wheel drive!!! (which actually isn't great). The RC receiver outputs for 2 & 4 go to the motor controllers, which can be configured by computer. There's a tiny acceleration/deceleration delay so that he doesn't slam the chains, which seems about right. The motor controllers are also throttled at about 1/2 power because I didn't want to have a short or something and have the frame got out of control. That'll have to be increased for real. The speed's about right on smooth floor, but on carpet it's a bit slow, and it'll be even slower with another 30 pounds of Lego bricks on it!

Gory Details

The frame is mostly aluminum t-slot, with some brackets and stuff to help out. I want him to eventually do "2-3-2", which is going from two legged mode to three legged mode and back. Because of that his shoulders have a round bearing. The insides are connected to a huge t-slot beam, and the outside edges have a smaller bar connecting them (to keep them in sync - theoretically).

Along with that theme, the center leg would need to retract, so the center foot is on a single t-slot rail, hooked up to a linear bearing (above the middle plate, behind the battery in the video). At that point there are two fixed rails that go up (the ones sticking out the top). The center foot bearing can slide all the way to the top.

However, I've put a stop on the center leg for now, and the bottom ankle hinge is fixed. The outside legs are hinged in the middle, but they've got lots of wheels, unlike the front foot. With the current single wheel, the front foot would fall over if the ankle wasn't rigid.

The single foot was really loud when I hooked it directly up to the battery, but the noise on the frame isn't bad at all with both motors regulated and running more slowly.

The ankles are currently 3 layers of plywood glued together. Originally the front ankle was a single layer, but that snapped pretty quickly and I rebuilt it. There's a brass collar through the wood where the ankle pivots on the foot.

With the flexible ankles I figured I needed front & rear wheels, and for various reasons I chose 3" roller blade wheels. Of course, it's more stable if the foot has 4 wheels, so I did that too. And then I powered both axles. The chain runs under the foot, so I probably don't want to get on very rough or dirty surfaces.

What I realized a bit belatedly is that with 4 wheels in the corner of the foot, it's kind of "hard" to make him turn like a tank. Quite a few wheels have to slip sideways for a tank turn to work. It seems "good enough" for now though, and isn't too bad if I make wide turns. OTOH, if I rapidly switch directions, he wiggles like when Kenny Baker throws R2 around.

Test Results

I took it to work twice to see where the problems were. Astromechs have a terrible problem with the front foot and bumps. He did OK at first, but the impact seemed to weaken the joints, so he started folding up the legs. If the front wheel stops, the power of the side legs is pretty impressive, they try to keep pushing.

The first day, I'd zip-tied the beam connecting the outer hubs together to the top, so that he'd be stuck in three legged position. However the beam is actually 3 parts because it has to avoid the center mount for the center foot. Those joints proved weak on repeated banging into sidewalk cracks, and eventually the legs started folding up.

In the car I'd tied him down with bungies to a tie point in the middle, which worked going to work, but on the way back I basically (gently) turned a corner and the bungies pulled all the feet together, his legs folded up and he fell over.

That also knocked the battery loose and trying to get him back upright I bumped one of the wires into the other terminal. Fortunately it was only a momentary contact (melting the wire helped), but I probably want to do something about capping the terminals. I'll have a better mount when I do it "for real", but better safe than sorry (it's an 18 Ah 12V lithium iron phosphate battery, like Tesla uses, but tinier).

2nd Test

I drilled a hole in the shoulder hubs and bolted them together to keep the feet from rotating. Because the hubs are currently wood, it didn't work too well, but it helped my zip-tie thing.

The rear feet frames are also laser cut, and seem OK that way. The Lego brick shell fits nicely over the frames, almost perfect on the first try (close enough I'm not going to fix it). The wood's a tiny bit flexible, but way lighter than aluminum. I put one of the Lego foot shells on for the second test to make sure that the Lego didn't vibrate apart.

I zipped by the company store to our makerspace to show off the parts I'd made on the laser cutter. Even when he's just a bare frame people stop and gawk, and most recognize what he'll eventually be when he gets his skin on!

The second test seemed to go way better, but I noticed that the legs "wander" in and out a little (toward and away from the body).

I'm not quite sure what happened, but on the way back to the car, one of the outer shoulder hubs cracked and he started doing the splits. That was sort of expected, we've planned to replace the plywood with aluminum there, but it wasn't great timing, he was in the driveway, and a delivery truck chose that instant to start "beep beep beep" backing up.

I didn't really want the frame run over, so I picked him up to carry to the sidewalk, and the other leg fell off! The four bolts near the hub had worked themselves loose a tad apparently. That's not really a problem, but I'm clearly going to have to find a better way to secure it. I'd noticed that several other bolts seem to have vibrated themselves a tad looser.

At home I took a closer look, and the brass inserts in the ankles are also working their way loose in the wood. I'd already thought about making the ankles out of aluminum, but that seems more likely.

I don't quite get chills watching the naked frame run around, but my wife thinks he's already getting a personality.

I can't wait to get the bricks on and get him back together. I sure hope I can make ECCC in March.